THE DIVERSITY AND OCCURRENCE OF POTASSIUM-DOMINANT AMPHIBOLES

Abstract

Currently, the Commission on New Minerals and Mineral Names (CNMMN) of the International Mineralogical Association (IMA) recognizes seven amphibole species where potassium is the dominant A-site cation. These include potassic-ferrisadanagaite, potassic-fluororichterite, potassic-magnesiosadanagaite, potassicpargasite, potassicsadanagaite, potassic-chloropargasite and potassicleakeite. However, as a result of an evaluation of compositions of Cl-bearing amphibole samples from iron-oxide-rich ore systems and a comparison with data in the reviewed literature, fourteen additional naturally occurring potassium-dominant amphiboles can be justified from selected published chemical data using recommended protocols for normalization. In keeping with accepted amphibole nomenclature, these new amphibole end-members would be "potassichastingsite", "potassic-chlorohastingsite", "potassic-ferropargasite", "potassic-chloro-ferropargasite", "potassic-aluminosadanagaite", "potassic-chlorosadanagaite", "potassic-chloro-ferro-edenite", "potassicrichterite", "potassic-ferrorichterite", "potassic-fluoro-magnesiokatophorite", "potassic-ferritaramite", "potassic-magnesio-arfvedsonite", "potassic-fluoro-magnesio-arfvedsonite" and "potassickornite". In several of the potassium-dominant amphiboles evaluated, chlorine is an important OH-site component, even where it is not the dominant anion, and the optional modifier "chlorian" is appropriate. Indeed, various species of both Cl-rich and K-rich ferropargasite and hastingsite (both sensu lato) are characteristic amphiboles of certain iron-oxide-rich hydrothermal systems associated with alkali–chloride metasomatism. At one locality, another new chlorine-dominant amphibole, although lower in potassium, seems present: potassian "chlorosadanagaite". In addition to mineralization related to hypersaline fluids, the "potassic-" amphiboles are otherwise widespread in diverse igneous, metamorphic, and metasomatic environments. It is hence desirable and justifiable, notably for those cases where results of chemical analyses are accompanied by data on crystallographic and physical properties, to confer species status to these amphiboles.